Carbon dioxide, which is the most abundant of the six major greenhouse gases causing global warming, is an acidic gas, and there is a restriction on the number of facilities permitted to discharge carbon dioxide in large quantities. Carbon dioxide is generally generated as a result of burning fossil fuels and thus is mainly generated in industrial processes in which energy is generated or consumed in large quantities.
According to a strategy for actively responding to climate changes proposed by the International Energy Agency in 2012, it is expected that carbon dioxide, which needs to be reduced by about 22% by 2050 in order for human beings to survive, will be reduced using carbon capture and sequestration (CCS) technology. CCS technology will also need to be procured domestically in order to reduce by 2020 the expected quantity of greenhouse gas emissions or the 2020 Business as Usual (BAU) scenario by 30%.
CCS technology includes the three steps of capture/compression, transport, and sequestration of carbon dioxide. Among these steps, the method of capturing carbon dioxide is the most costly and thus has received the greatest focus in terms of their technical development. Several techniques for capturing carbon dioxide have been developed since the 1900's, some of which have been made available commercially. Analysis results show that carbon dioxide can be captured in large quantities, and the most economical method for capturing carbon dioxide in large quantities is the liquid absorption method. The liquid absorption method is mainly divided into the physical absorption method and the chemical absorption method, and the chemical absorption method associated with the present invention that are available commercially are listed in Table 1.
TABLE 1Product Chemical Process operatingnamessolvents usedconditionsChemicalMEA2,5n monoethanol 40° C., ambientsolventsamine and inhibitorsintermediate pressuresAmine 5n monoethanol 40° C., ambientguardamine and inhibitorsintermediate pressuresEconamine6n diglycol amine80 to 120° C., 6.3 MPaADIP2-4n diisopropanol 35 to 40° C., >0.1 MPaamine2n methyldiethanol aminea-MEDA2n methyldiethanol amineFlexsorbHindered amineKS-1, KS-2, Hindered amine andKS-3promotersBenfield Potassium carbonate 70 to 120° C., andand catalysts. Lurgi & 2 to 2.7 MPaversionsCatacarb processes with arsenic trioxide
Among these liquid absorption methods, the most commonly used liquid absorption methods include an alkanolamine method (in which monoethanolamine, diethanolamine, triethanolamine, etc. are used) and a Benfield method in which potassium carbonate is used. The alkanolamine method involves utilizing various types of alkanolamines that absorb carbon dioxide after the alkanolamines are mixed with water to prepare a 20 to 30% by weight solution. Because of its ability to rapidly absorb carbon dioxide, the alkanolamine method has been available for commercial use since the 1970's.
According to the alkanolamine method, a regeneration reaction requires injection of a very high amount of heat energy when the regeneration reaction is performed after a carbamate is formed in a form of a combination of an alkanolamine and carbon dioxide. Therefore, due to the high amounts of energy required to regenerate absorbents in these conventional amine-based absorption processes, there has been a demand for a reduction in capturing costs.
Referring to conventional carbon dioxide absorption processes, an exhaust gas enters a direct contact cooler so that the exhaust gas is cooled by refluxed water vapor. In this case, the exhaust gas is compressed in an air blower in order to cope with a pressure drop caused by reflux of the vapor and is allowed to flow in an absorption unit in a countercurrent direction with respect to an absorbent. The absorbent flowing in the opposite direction of the exhaust gas chemically reacts with carbon dioxide present in the exhaust gas. A CO2-lean gas enters a washing part of the absorption unit. In this case, water and the absorbent are separated at the washing part and then return to the absorption unit, and the washed gas is released into the air.
A CO2-rich gas is pumped from the absorption unit into a lean/rich cross heat exchanger. In the cross heat exchanger, a CO2-rich solution is heated and a CO2-lean solution is cooled. To regenerate a solvent, the CO2-rich solution is heated in a reboiler using low-pressure steam, and water and the absorbent in the mixture are evaporated by heating. A vapor of the absorbent and the steam enter a regenerator from the reboiler. In the regenerator, carbon dioxide is separated, and the vapor flows upwards at the time that the solution flowing downwards is heated. Some of the vapor and the carbon dioxide gas enter a washing part of the regenerator. In the washing part, the steam is condensed, the carbon dioxide is cooled, and condensed water returns to the regenerator. Also, the CO2-lean solution leaves the reboiler, and is cooled in the cross heat exchanger. The solution is cooled further prior to being returned to the absorption unit.
As other absorbents for capturing carbon dioxide and the related process technology, liquid absorbents for capturing carbon dioxide (Brand name: KIERSOL; registered Trademark Nos. 40-2011-0046524 and 40-2011-0046525), each of which includes potassium carbonate as a main component, and the related process technology in registered Korean Patent Nos. 1157141, 1316543, 123938, and the like were independently developed. In processes using such absorbents, the energy consumed to regenerate the absorbents is approximately 2.5 GJ/tCO2, which is at least 20% lower than that of the KS-1 process of Mitsubishi Heavy Industries. Ltd. (MHI, Japan), which currently possesses the best technology in the world (regeneration energy: 3.2 GJ/tCO2). Also, since the absorbents are influenced less by sulfur oxide or halogen compounds included in small amounts in the combustion exhaust gas, it is possible for the absorbents to compensate for shortcomings of other absorbents currently available, such as the need to continuously supplement an absorbent during operation of the process, and to reduce operation costs.
Registered Korean Patent No. 712585 discloses a method of separating and recovering carbon dioxide from by-product gases produced at a steel mill using a chemical absorption method. Here, the technology using a low-graded array produced at the steel mill was applied to processes of absorbing carbon dioxide from the gas into a chemical absorption solution and heating the chemical absorption solution to separate carbon dioxide.
To solve problems of water shortage and energy depletion caused by global warming, methods of desalinating seawater, which accounts for most of the water on the Earth's surface, have also been studied. Distillation used in the Middle East and reverse osmosis widely used in the US, Japan, etc. are representative methods. However, reverse osmosis also consumes large amounts of energy because it involves using a high-pressure pump to obtain produced water. To address this problem, devices for recovering energy from high-pressure condensed water have been developed. Technology for producing electricity using a system similar to forward osmosis (FO) capable of dramatically reducing energy consumption, pressure-retarded osmosis (PRO) also known as energy generation technology, and electrodialysis (ED) in which anion exchange membranes and cation exchange membranes are installed alternately between negative electrodes and positive electrodes, but using the same system as forward osmosis (FO) in which there are two pairs each of flows supplied to the system and flows discharged from the system, and when a space between the ion exchange membranes is filled with seawater and river water, electrons are transferred from the negative electrodes to the positive electrodes with movement of ions due to a voltage difference generated by a difference in salinity between the seawater and the river water has been studied by Dr. Braun's team in Belgium, Dr. Hameler's team in the Netherlands, and the like.
Dr. Hameler's researcher team reported that mixing energy is released when two fluids having different compositions are mixed, and that although there is no technology for obtaining this energy from gases and liquids, when carbon dioxide mixed with combustion gas in the air is regarded as an energy source, it has a total annual worldwide capacity of 1,570 TWh. They also reported that pairs of porous electrodes, which include an anion-selective electrode and a cation-selective electrode, are used to obtain mixing energy from discharging gases including carbon dioxide, and electric energy is obtained between the selective porous electrodes when a flushing electrolyte is allowed to flow alternately with carbon dioxide or air. In addition, they reported that efficiency of this process is 24% when the electrolyte is non-ionized water and is 32% when the electrolyte is 0.25 M MEA. When the MEA solution is used as the electrolyte, an amount of maximum average energy is 4.5 mW/m2, the value of which is remarkably higher than 0.28 mW/m2 when water is used as the electrolyte.
Mixing two solutions of different composition leads to a mixture with a lower Gibbs energy content compared to the original two solutions. This decrease in the Gibbs function indicates the presence of mixing energy that can be harvested when a suitable technology is available. Up until now, the use of the mixing process as a source of energy has only been exploited for mixing of aqueous solutions with a different salinity. Mixing freshwater from rivers with seawater typically has an available work of −3 kJ per L of freshwater. Several technologies are being developed to exploit this source of energy using semipermeable membranes, ion-selective membranes, double-layer expansion and ion-selective porous electrodes. The latter technology is based on the use of capacitive electrode cell pairs; similar to those used in supercapacitors or in capacitive deionization (CDI) for water desalination. Another approach uses a fuel cell in which dry air at the cathode side is used to maintain operation as an electrochemical concentration cell.
Also, the researcher team has investigated the possibility of obtaining energy from the emission of carbon dioxide. Wherever hydrocarbon fuels or biomass are combusted, i.e. converted to CO2 and water, emissions containing high CO2 concentrations (5%-20%) compared to air (0.039%) are produced. This means that mixing combustion gas with air is an unexplored source of energy. To harvest this energy source the researcher team has suggested to contact both the CO2 emission and air with an aqueous electrolyte. In aqueous solutions, CO2 reacts with water to produce carbonic acid that itself dissociates into protons (W) and bicarbonate (HCO3−), which can further dissociate at high pH to carbonate ions (CO32−). An increase of the CO2 pressure in the gas leads to an increase of the concentration of the ions in the aqueous solution. The resulting difference in the ion concentration between the air-flushed solution and the CO2-flushed solution can be used to gain electrical energy. Here, the researcher team has addressed the feasibility of obtaining additional energy from mixing CO2 emissions and air.
The experimental setup consisted of two tanks containing the electrolyte. One tank was flushed with air while the other was flushed with 100% pure CO2 gas. Each tank was connected to the capacitive cell via a peristaltic pump. Each of pumps, T connectors, and valves are configured to prevent backflow, a pH probe is installed in the inlet of a capacitive cell, The outlet of both pumps was connected to the inlet of the capacitive cell via a T shaped connector. In this case, Cell potential under open circuit conditions, or in a closed circuit via an external load, was measured with a multimeter, with the anion exchanging electrodes connected to the ground of the multimeter. In a capacitive cell composed of two capacitive electrodes, one electrode is covered with a cation exchange membrane (CEM) and the other is covered with an anion exchange membrane (AEM). A cell used in the experiments is formed by stacking a plurality of layers so that a flat flow passes through the cell, and is composed of (1) an aluminum plate used as an exterior plate, (2) a graphite plate socket having a hollow poly(methyl methacrylate) (PMMA) plate used as a current collector, (3) a silicone gasket configured to seal the cell and form a space for the capacitive electrodes, (4) the capacitive electrodes made of a graphite foil current collector coated with an activated carbon layer, (5) a CEM selective to cations (protons), (6) a Teflon gasket configured to form a space for a spacer, (7) a polymer spacer configured to guide the flow of a fluid with a membrane, and (8) an anion exchange membrane layer selective to anions (bicarbonate ions).
A porous carbon electrode was prepared by mixing activated carbon powder in a binder solution, and pretreatment was performed by immersing the carbon electrode in a carbon dioxide-containing solution or an MEA solution. An anion membrane and a cation membrane were immersed in a 0.25 M hydrochloric acid solution in the case of the CEM, and in a 0.25 M potassium bicarbonate (KHCO3) solution in the case of the AEM for 24 hours, during which an immersion solution was replaced twice. A polymer spacer was used to form a flow path.
Two solutions were supplied to a pump through a spacer channel between the two ion exchange membranes, and a flow of the CO2-flushed water and a flow of the air-flushed water were supplied alternately through the device in all experiments. These two steps constituted one cycle. The water was dissociated from different salts. In this case, the temperature was 20° C., and the setup operated at atmospheric pressure. It is possible to produce electricity by connecting the two electrodes through an external load Rext (Ω), allowing electrons to flow between the electrodes. When exposed to the CO2-flushed water, the membrane potential will drive electrons from the anion specific electrode to the cation specific electrode. This transport of electronic charge leads to an excess charge in each electrode. To maintain electroneutrality, this excess charge is compensated by counter ion adsorption at the electrode surface, until equilibrium is reached between the membrane potential and the double layer potential and the cell voltage becomes zero. When the CO2-flushed solution is replaced by the air-flushed one, the new membrane potential will reverse these processes and drives the ions out of the electrodes, back into the flowing solution, until the system reaches its new equilibrium where again the cell potential is zero. This clear zero cell potential is typical for an energy-producing mode of operation of the cell. However, under open circuit condition there is no charge transport and there is thus no possibility for the electrode double layer potential to equilibrate with the membrane potential. As a consequence, the potential will only change as the result of the change in membrane potential, but it is not self-evident that a zero cell potential will be reached because the electrode potentials remain constant. Cycles can be repeated by alternatingly pumping the two solutions. Both the air flushed and CO2 flushed solutions were prepared by gas sparging, as this is a simple technology easily applied in the laboratory. However, sparging is an energy intensive operation that has been extensively studied in wastewater treatment. There, the specific aeration efficiency is in the range 0.6-7.5 kg O2/kWh depending on the technology applied. Even using the most efficient aeration technology we estimate we need around 300 kJ per kg CO2 for a single solution. This calculation shows that the use of sparging to contact the gases with the electrolyte consumes more energy than is produced. The researcher team configured an electricity-generating device using ion to experimentally prove a principle of a process of obtaining electric energy in which dissolved carbon dioxide is dissociated into protons and bicarbonates and then diffused to different electrodes due to ion selectivity, and thus the resulting membrane potential leads to spontaneous production of electric current.
As seen in registered Korean Patent Nos. 131136, 1291768, and 1318331, research using pressure-retarded osmosis (PRO) in which electricity is generated by generating osmotic pressure 26 times higher than atmospheric pressure by using a concentration difference to pass freshwater toward seawater through a semipermeable membrane installed therebetween and reverse electrodialysis (RED) in which only specific ions selectively pass through a membrane has been conducted.
However, no research about production of electricity by using a carbon dioxide absorption tower to apply an absorption solution, in which carbon dioxide included in a combustion gas is absorbed, to an electricity-generating device using ion using seawater and freshwater has been attempted.
More specifically, at present, in the case of process technology for capturing carbon dioxide, technology development has focused on improvements in material performance and process efficiency for the past 40 years in order to reduce the energy required to regenerate chemical materials, and in the case of ion generating technology using a salinity difference, technology development has focused on improvements of membrane performance and a membrane module system in order to overcome the limitation on the amount of electricity that is generated with the low salinity of seawater (3.5%). To address the difficulty of developing these two technologies, processes of capturing carbon dioxide are operated through a combination of technology for capturing carbon dioxide and ion generating technology using a salinity difference. In this case, the heat energy necessary for reclamation is not required but a carbon dioxide absorption solution actually becomes a base material for generation of electricity, and a high amount of electric current is obtained with a high salt concentration difference. Therefore, it is thought that when these two problems are dramatically solved, a technical paradigm for solving global warming will be achieved.